Plasma fatty acids, prostaglandin F-2 alpha metabolite, and reproductive response in postpartum heifers fed rumen bypass fat

An experiment was conducted to determine whether feeding rumen-protected fa tty acids (FA) to postpartum heifers would increase plasma concentrations o f linoleic acid and PGF(2 alpha) metabolite (PGFM), shorten the interval fr om calving to first increase in plasma concentrations of progesterone (P-4) , and increase pregnancy rate relative to controls. Hereford x Angus heifer s (346 kg) were assigned randomly to treatments containing either lipid or barley supplemented diets for the first 30 d postpartum. Lipid was .23 kg.h eifer(-1).d(-1) of calcium salts of FA (CSFA; n = 20), and an isocaloric am ount of barley served as the control (n =19). Supplements, with .23 kg of b arley as a vehicle, and a basal diet of meadow and alfalfa hays were pen fe d to heifers (5/pen). Heifers were bled on alternate days (d 1 to 30) and t wice weekly (d 30 to 2 wk after first estrus) for RIA of plasma PGFM and Pq , respectively. Weight percentage of major FA in plasma on d 1 and 7 was de termined with gas chromatography. First behavioral estrus was detected by u se of intact bulls and confirmed toy an increase in plasma Pq On d 7, but n ot d 1, plasma from heifers fed CSFA had altered proportions of major FA (P < .01), including an increase in linoleic acid compared with those of cont rols (29.1 vs 25.6% of total FA; SE = .75; P <.01). Analysis of variance of contrast variables revealed an effect of treatment on direction of change in PGFM, from d 3 to 5 (P < .01). By d 7 and on d 9, plasma concentrations of PGFM. were greater in heifers fed CSFA than in controls (P = .02 and P = .06, respectively). There was no difference in plasma concentration of PGF M between treatments on d 1, 3, 5, 11, 13, and 15 postpartum (P = .80, .17, .52, .82, .46, and .77, respectively). Days to first estrus with ovulation , pregnancy rate, and calving interval were not affected by treatments (P = .58, .52, and .24, respectively). Although supplemental lipid fed to primi parous beef heifers increased plasma levels of linoleic acid and production of PGFM in the early postpartum period, it did not improve the fertility o f these heifers in the subsequent breeding season.